Photosensitisers, such as toluidine blue O, act as light-activated antimicrobial agents. Although they may have no antimicrobial activity at low concentrations in the dark, when irradiated with light of a certain wavelength (such as 633 nm for toluidine blue O) they are able to kill a wide range of microbes. Killing is thought to be due to the singlet oxygen produced on irradiation of the compound. There is considerable interest in enhancing the activity of existing photosensitisers. The present invention focuses on one method of achieving this.
US 2005/0058713 describes that singlet oxygen production by a photosensitiser (zinc phthalocyanine) is enhanced by covalently linking it to gold nanoparticles (see also Duncan C. Hone, Peter I. Walker, Richard Evans-Gowing, Simon FitzGerald, Andrew Beeby, Isabelle Chambrier, Michael J. Cook, and David A. Russell. Langmuir 2002, 18, 2985-7). However, this increase in singlet oxygen generation has been reported to be due, at least in part, to the presence of tetraoctylammonium bromide—a reagent used in the preparation of the phthalocyanine-nanogold. The authors concluded, therefore, that the singlet oxygen generating system was, in fact, a three-component system consisting of nanogold, the phthalocyanine and the tetraoctylammonium bromide. Although the phthalocyanine/nanogold/tetraoctylammonium bromide was found to increase singlet oxygen generation, it was not demonstrated that these particles were able to kill either mammalian cells or microbes.
Nanoparticle suspensions are inherently unstable, and the nanoparticles tend to associate, or clump together. Two methods are used to counter this. One is ligand-stabilization, which is employed, for example, in US 2005/0058713. The other is charge-stabilization.
The present inventors have found that, surprisingly, simple mixing of charge-stabilized metallic nanoparticles with a photosensitiser results in enhancement of antimicrobial activity.
The present inventors have also found that, surprisingly, metallic nanoparticle-ligand-photosensitiser conjugates, in which a photosensitiser is directly bound, via the ligand, to ligand-stabilised nanoparticles, have enhanced antimicrobial properties.